The two surfaces you are trying to merge are disconnected from one another. They should overlap, or at least share edges.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.4.0.0 (26.4.0.0)
Syntax
C#
public static FailureDefinitionId SiteMergeDisconnected { get; }